Effect of local dipole moments on the structure and lattice dynamics of K_0.98Li_0.02TaO₃

We present high-energy x-ray (67 keV) and neutron-scattering measurements on a single crystal of K_1−xLi_xTaO₃ for which the Li content (x=0.02) is less than x_c=0.022, the critical value below which no structural phase transitions have been reported in zero field. While the crystal lattice does remain cubic down to T=10 K under both zero-field and field-cooled (E≤4 kV/cm) conditions, the Bragg peak intensity changes significantly at T_C=63 K. A strong and frequency-dependent dielectric permittivity is observed at ambient pressure, a defining characteristic of relaxors. However an extensive search for static polar nanoregions, which is also widely associated with relaxor materials, detected no evidence of elastic neutron diffuse scattering between 300 and 10 K. Neutron inelastic scattering methods were used to characterize the transverse acoustic and optic phonons (TA and TO modes) near the (200) and (002) Bragg peaks. The zone-center TO mode softens monotonically with cooling but never reaches zero energy in either zero field or in external electric fields of up to 4 kV/cm. These results are consistent with the behavior expected for a dipolar glass in which the local polar moments are frozen and exhibit no long-range order at low temperatures.